1. Field of the Invention
The present invention relates generally to medical methods and devices, and more particularly to a method and apparatus for forming vascular prostheses from host tissue sources.
Coronary and peripheral atherosclerosis are characterized by partial or total occlusion of the arteries resulting from the accumulation of lipids, smooth muscle cells, connective tissue, and glycosaminoglycans on the arterial wall. Atherosclerosis of the coronary arteries is a particular problem and can cause angina and myocardial infarction (heart attack). Although many coronary lesions can be treated with percutaneous techniques, such as angioplasty and atherectomy, more tortuous and severely diseased arteries frequently require surgical intervention and bypass, commonly referred to as coronary artery bypass graft (CABG) surgery.
CABG surgery relies on the surgical attachment of a vascular graft to bypass the arterial occlusion in order to restore blood flow to the coronary vasculature. The nature of the vascular graft can have a significant impact on the ultimate success of the procedure. A preferred vascular graft is formed from autologous internal mammary artery (IMA), where the resulting grafts have a patency rate approaching 95% ten years following the procedure. The use of IMA grafts, however, is limited by their length, and the need to harvest the artery from the patient can result in post-surgical complications. The autologous saphenous vein is a second common source for vascular grafts. While generally available in the necessary lengths, the saphenous vein is not ideally suited for replacement as an arterial vessel, and patency rates at ten years are often below 50%. Moreover, removal of the saphenous vein from the leg can also cause post-surgical complications.
Because of the limitations on autologous vascular sources, a variety of synthetic and non-autologous biological prostheses have been proposed. Common synthetic prostheses are formed from Dacron(copyright) and PTFE, and can perform well when employed in larger diameters, i.e., above 6 mm. Smaller synthetic prostheses, however, occlude at a relatively high rate. Non-autologous biological conduits which have been utilized as vascular prostheses include human umbilical vein grafts and bovine internal mammary arteries. Synthetic grafts have also been seeded with human and other mammalian cells or proteins, e.g., collagens, in an effort to improve their long-term patency rate. Presently, however, none of these approaches has demonstrated long-term patency, particularly in smaller diameter grafts.
Of particular interest to the present invention, preparation of vascular prostheses from autologous pericardium has been proposed. Pericardial tissue is harvested from the patient and formed into a tubular graft by suturing along a longitudinal line. While promising, the use of sutures can result in an irregular seam which, in turn, can cause turbulent blood flow and result in clot formation. Moreover, such grafts are unsupported and subject to kinking and collapse. The grafts further lack an inherently round geometry and are subject to dimensional changes, e.g., elongation and aneurysmal formation. Because of the dimensional uncertainty, it is difficult to match such grafts to the precise dimensional requirements of the particular application, e.g, caliber and length. The suturing of vascular prostheses from pericardium is labor intensive and time consuming, and the resulting structures are subject to rupture and other structural failure. Thus, the outcome of using sutured pericardial tissue grafts is uncertain at best.
A significant improvement over such prior autologous pericardial graft structures is disclosed in copending application Ser. No. 08/580,582, filed on Dec. 29, 1995, and assigned to the assignee of the present application, the full disclosure of which is incorporated herein by reference. In that copending application, a prosthetic graft is formed by wrapping a sheet of tissue over a first helical frame which is optionally supported on a mandrel. A second helical frame is then placed over the tissue-wrapped mandrel to complete the graft structure. No suturing is necessary, and the resulting structure is dimensionally stable, available in a variety of lengths, and biologically compatible.
Despite such advantages, the graft structures described in the copending application can be cumbersome to fabricate, particularly when the fabrication is carried out in the operating room after harvesting of the tissue and before performance of a CABG or other grafting procedure. For example, placement of the outer helical frame over the tissue-wrapped inner frame can be difficult, particularly when trying to properly align adjacent turns of the outer helical frame between corresponding turns of the inner helical frame. Removal of the completed graft structure from the assembly mandrel can be difficult. Both the helical frames and the tissue are at risk of damage, and it is difficult to achieve repeatable, consisten graft structures.
For these reasons, it would be desirable to provide improved methods and apparatus for forming tubular prostheses from patient tissue over tubular support frames. It would be particularly desirable to provide methods and apparatus which permit the rapid placement of such helical outer frame components over an assembly mandrel with a high degree of accuracy and repeatability, and which further permit and facilitate removal of the fully formed prosthesis from the assembly mandrel without damage to either the tissue or the helical support structures. Such methods and systems should be suitable for preparing tubular prostheses having a wide range of diameters and lengths, and should be relatively easy to use while minimizing any chance of improper use. The present invention will address at least some of the objectives set forth above.
2. Description of the Background Art
U.S. Pat. No. 4,502,159, describes a vascular prosthesis made by suturing glutaraldehyde-treated pericardial tissue along a longitudinal seam. SU 1217362 (Abstract) describes reinforcing arteries by securing pericardial tissue over the artery. U.S. Pat. No. 3,562,820, describes forming tissue-containing prostheses over removable mandrels. The use of glutaraldehyde and other agents for treating tissue and prosthetic devices to reduce antigenicity is described in U.S. Pat. Nos. 3,988,782; 4,801,299; 5,215,541, and Brazilian applications 89/03621 and 90/03762. U.S. Pat. No. 4,539,716, describes the fabrication of an artificial blood vessel from collagen and other natural materials. U.S. Pat. Nos. 3,894,530 and 3,974,526, describe the formation of vascular prostheses from the arteries or veins present in the umbilical cord. U.S. Pat. No. 5,372,821, describes the use of tissue for forming artificial ligament grafts for use in orthopedic procedures. U.S. Pat. No. 3,408,659, describes the preparation of vascular artificial prostheses from other body lumens. French application FR 2,714,816, (Abstract) discloses a helically supported vascular prosthesis. A number of medical literature publications describe the use of vascular prostheses formed form tissue. See, for example, Rendina et al. (1995) J. Thorac. Cardiovasc. Surg. 110:867-868; Hvass et al. (1987) La Presse Medicale 16:441-443; Allen and Cole (1977) J. Ped. Surg. 12:287-294; and Sako (1951) Surgery 30:148-160. Other patents and published applications relating to synthetic vascular grafts include U.S. Pat. Nos. 4,728,328; 4,731,073; 4,798,606; 4,820,298; 4,822,361; and 4,842,575; and PCT publications WO 94/22505 and WO 95/25547. Patents and published applications relating to kits for preparing replacement heart valves from pericardial and other autologous tissue sources are described in U.S. Pat. Nos. 5,163,955; 5,297,564; 5,326,370; 5,326,371; 5,423,887; and 5,425,741.
The present invention provides improved methods, systems, and apparatus for forming tubular prostheses from animal tissue, usually autologous tissues from the patient who is to receive the prosthesis. The tubular prostheses are of the type which comprise an inner tubular frame, usually an inner helical member, a sheet of tissue wrapped around the inner tubular frame, and a second helical frame disposed over the wrapped tissue. The tissue is usually wrapped around the inner frame more than once, and the resulting tissue xe2x80x9csleevexe2x80x9d is captured between the inner and outer frames in the manner described in copending application Ser. No. 08/580,582, the full disclosure of which has previously been incorporated herein by reference.
The methods of the present invention are particularly suitable for performance in the operating room as part of a procedure for implanting the prosthesis. Thus, the methods herein will usually be performed at a point after the tissue has been harvested and prior to the implantation, and it is thus highly desirable that the methods be expeditious, relatively simple to carry out, and present minimum chance for error. The methods, systems, and apparatus of the present invention are very simple to use, reliably form the desired vascular prosthesis with proper alignment of the various components, and present minimum opportunity for error. In particular, the present invention provides for the simple and accurate placement of both an inner helical member and an outer helical member while facilitating wrapping of the tissue therebetween. The present invention further facilitates removal of the assembly mandrel so that the final prosthetic structure can be removed from the assembly mandrel with minimum risk of damage.
In a first aspect, the method of the present invention for forming a tubular prosthesis comprises providing a sheet of tissue, typically autologous tissue which is prepared by the procedures described below. The sheet of tissue is wrapped over a tubular inner frame member which, in turn, is disposed over a mandrel. A helical outer frame component is then placed over the wrapped tissue to form the tubular prosthesis. The mandrel is then collapsed and axially withdrawn from the tubular inner frame member to leave the tubular prosthesis ready for trimming prior to use.
In preferred embodiments of this method, the wrapping step may comprise placing an edge of the tissue over the tubular inner frame member, and thereafter rotating the mandrel to wrap at least one and one-half turns of the tissue thereover, usually from one and one-half to three turns. Preferably, two turns of the tissue will be wrapped in order to provide a prosthesis which does not need suturing to prevent leakage. Preferably, although not necessarily, the inner tubular member will comprise a helical frame member having a pitch which is the same as that of the outer helical frame member so that the inner and outer helical members are aligned in parallel over the length of the prosthesis.
Placement of the helical outer frame component over the wrapped tissue will preferably comprise rotating the mandrel to draw the helical outer frame component over the wrapped sheet of tissue, and preferably further comprise guiding the helical outer frame component as it is being drawn over the wrapped sheet of tissue. The guiding step will usually comprise rotating a lead screw, where the helical outer frame component is received in and advanced by a helical groove on the lead screw as it is rotated. The mandrel collapsing step will usually comprise withdrawing an inner support rod from an outer collapsible shell, e.g. at least two axial runners which are aligned over the cylindrical support rod.
In a second aspect, the method of the present invention for forming a tubular prosthesis comprises providing a sheet of tissue and wrapping the sheet of tissue over a tubular inner frame. The wrapped tubular inner frame is then axially aligned with a helical outer frame component, and an end of the helical outer frame component coupled to the wrapped tubular inner frame. By then rotating the tissue wrapped inner frame, the helical outer frame is drawn over the outer surface of the tissue.
In preferred embodiments of this aspect, the method further comprises guiding the helical outer frame component as the tissue-wrapped inner frame is rotated so that successive turns of the component are laid over said tissue-wrapped inner frame at a substantially even pitch. The guiding step conveniently comprises rotating a lead screw which is axially aligned with the tissue-wrapped inner frame and the helical outer frame component, where the helical outer frame component is received in and advanced by a helical groove on the lead screw. Typically, the tubular inner frame is disposed over a mandrel, and the rotating step comprises rotating the mandrel. Optionally, the mandrel may be radially collapsed in order to facilitate removal of the completed tubular prosthesis from the mandrel.
In a first aspect of the apparatus of the present invention, a tubular prosthesis assembly system comprises a base, an elongate lead screw rotatably mounted on the base, an elongate support for a helical outer frame component mounted on the base and parallel to the lead screw, and a prosthesis assembly mandrel removably mounted on the base and parallel to the lead screw and the elongate support. The prosthesis assembly mandrel and the elongate support member are disposed on opposite sides of the lead screw so that a helical outer frame component (which can be disposed over the elongate support but which does not form part of the tubular prosthesis assembly) can pass from the elongate support, over the lead screw, to the prosthesis assembly mandrel. The assembly further comprises a driver for synchronously rotating the lead screw and the prosthesis assembly mandrel in order to transfer the helical outer frame component from the elongate support to the prosthesis assembly mandrel. The lead screw and driver are configured to advance the axial position of the helical outer frame component over the exterior of the rotating prosthesis assembly mandrel so that the outer helical frame component will be precisely and accurately aligned in the desired pattern thereover, typically, at a pitch which matches that of an inner helical support member underlying tissue which is placed over the prosthesis assembly mandrel.
In preferred embodiments of the tubular prosthesis assembly system, the prosthesis assembly mandrel is radially collapsible. More preferably, the prosthesis assembly mandrel comprises a collapsible outer shell and an inner support rod, where removal of the inner support rod allows the outer shell to collapse to facilitate removal of a completed prosthesis therefrom.
In a second aspect of the apparatus of the present invention, a prosthesis assembly mandrel comprises an elongate cylindrical mandrel which is radially collapsible, a helical inner frame component is disposed over the elongate cylindrical mandrel. Typically, the elongate cylindrical mandrel comprises an inner support rod and an outer collapsible shell disposed thereover. Removal of the inner support rod axially from the outer shell permits the radial collapse of the shell. Typically, the prosthesis assembly mandrel has an outer diameter (when non-collapsed) in the range from 2 mm to 8 mm, and a length in the range from 15 cm to 50 cm.